Substrates for mounting and electrically interconnecting integrated circuit chips are formed by depositing blanket layers of chromium-glass cermet resistor material, chromium, copper, and chromium in sequence upon a ceramic substrate and subtractively etching any portions of the layers through overlying resist masks in order to form a structure of conductor lines and cermet resistors. In the past, chromium has been etched using mixtures of NaOH and KMnO.sub.4. This mixture, however, attacks the chromium in the cermet and changes its resistivity. Also, such mixtures attack positive resists of the phenol-formaldehyde novolak resin type so it is necessary to use negative resists. It is known to etch chromium in HCl-water mixtures but this is found to be effective only where chromium is in contact with copper. Therefore, after the edges of the lower chromium layer which are in contact with the copper are etched away, a residual island of chromium remains in the center. In U.S. Pat. No. 2,230,156, a process for etching chromium in contact with copper for lithographic plates is disclosed which uses mixtures of one volume of aqueous 37% HCl with 3 to 6 volumes of a glycol containing more carbon atoms than hydroxyl groups. U.S. Pat. No. 2,687,345 discloses the etching of chromium in bimetal lithographic plates using mixtures of HCl and polyhydric alcohols containing CaCl to reduce the attack on the resist layer during the etching process and provide a faster etch rate.
It has been found that, although mixtures of glycols and dilute HCl will etch the chromium at ambient temperatures, when etching the lower chromium layer the exposed edges of the top chromium layer etch during the relatively longer times needed to etch the lower layer so that undercutting of the top layer occurs. The result is that a portion of the copper is exposed at the top edges and ends of the conductor lines. When the structure is tinned, the copper is wetted by the solder in these exposed areas and solder bridges over the resistors and between the conductor lines can form which render the structure inoperative. A concentrated HCl mixture of 50% by volume or more has been found to provide an etch time for the lower chromium layer which sufficiently minimizes the undercutting of the top chromium layer, however, these concentrated acid mixtures attack the cermet and cause unacceptable changes in its resistivity. Surprisingly, we have found that by using dilute HCl and water miscible organic solvent mixtures at an elevated temperature, both undercutting of the top chromium layer and changes in cermet resistivity are kept to acceptable amounts while achieving a satisfactory etch rate of chromium.